This invention relates generally to rotary serial character printers and more particularly to impact printers having a print wheel mounted on a movable carriage, commonly referred to as "daisy wheel" printers.
Daisy wheel printers conventionally have a printer carriage bearing a rotatable print wheel. The carriage is mounted on a frame structure for side-to-side movement along a platen to shift the print wheel from one print position to the next along a horizontal print line. Such printers include a print wheel drive mechanism for rotating the print wheel to angularly position a petal of the wheel in the print line. A striker mechanism mounted on the carriage strikes the petal to impact it against the platen and thereby create an image on paper of the character borne on the petal. Coordination of movement of the carriage, rotation of the print wheel and operation of the striker requires precise control of both position and timing of operation of the various elements of the printer. In a quest for ever-faster printing speeds, while retaining the inherently superior quality of impact-generated characters, a variety of rotary printer designs have been proposed.
Among early designs are U.S. Pat. Nos. 3,542,182 to Langenberger; 3,651,916 to Becchi; and 3,698,529 to Cattaneo. Each of these patents uses a step-type drive mechanism for advancing a movable carriage along a platen. The carriage mounts a print wheel, which is continuously rotated asynchronously of the carriage in one direction through either a gear or a pulley drive. All of these designs use some form of photoelectric or conductive synchronization circuitry for sensing print wheel position and actuating the striker at a proper instant relative to the angular position of the constantly rotating print wheel to print a preselected character. The primary difficulty with devices of this type is that striking a fast-moving petal causes the image to smear. Smearing reduces the quality of printed character at high print speeds, thereby limiting the maximum printing speed of continuously-moving wheel-type printers. Accurately controlling the operation of the striker is essential to avoid mistriking the fast-moving petals. It imposes very stringent response and timing requirements on the striker which also make it difficult to further increase printing speed. Hence, it would be preferable to slow or stop the wheel momentarily during striking. At the same time, it would be desirable to dispense with the need for extrinsic synchronization circuitry.
Later printers, such as those disclosed in U.S. Pat. Nos. 3,760,925 and 3,773,161 to Bossi are of the synchronous type, that is, printer carriage motion is mechanically synchronized with the motion of the print wheel, which is continuously rotated in a single direction. However, mechanically synchronizing the carriage and print wheel limits the choice of spacing between letters to a pitch equal to the pitch of the screw drive of the carriage. This arrangement interferes with proportionally spacing the letters and virtually precludes graphics. An additional disadvantage of rotating the print wheel in only one direction is that it must rotate as much as a full turn between characters. It is preferable to be able to rotate the print wheel in either direction, by at most half a turn, independently of movement of the carriage, to position a next character in the printing position along the platen. This is clearly incompatible with a mechanically-synchronized carriage and print wheel of the type disclosed in the Bossi patents.
More recent printer designs have returned to a fully asynchronous relationship between the carriage drive and the print wheel drive. U.S. Pat. Nos. 4,044,880 to Martin and 4,101,006 to Jensen, et al. each disclose a step motor driving the carriage from side to side through a belt and pulley arrangement. A second motor rotates the print wheel through a pair of bevel gears, one of which is slidingly mounted on a splined drive shaft of the print wheel motor. U.S. Pat. No. 3,908,809 to Beattie is similar, but uses a constant speed carriage drive motor in combination with a stepper motor which acts as an intermittent brake on the continuous translational movement of the carriage. In Martin and Beattie, the print wheel motor rotates continuously in one direction and printing occurs on the fly, with both the wheel and carriage moving. In Jensen, et al, the print wheel drive motor is a stepping motor but otherwise operates mechanically much like Martin. Both Martin and Jensen, et al. use a toothed magnetic emitter wheel and sensor to sense the angular position of the print wheel and control firing of the striker, similar to the aforementioned Cataneo and Becchi patents. Beattie suggests using a similar arrangement to monitor carriage position.
As mentioned above, operation in Martin, Beattie and Jensen, et al. of the carriage and print wheel motors is asynchronous. The carriage and print wheels are moved independently of one another. As the carriage moves, the bevel gear mounted on splined shaft of the print wheel step motor merely slides along the shaft. This mechanical arrangement is superior to those of prior asynchronous designs but still suffers from several disadvantages. First, the gears must be highly precise both in construction and in their positioning relative to one another. Properly meshing bevel gears necessarily involves a trade off between accuracy of print wheel position and vulnerability to backlash on one hand and friction or binding between the gear teeth on the other hand. Friction between the movable bevel gear and the splined shaft is also desirably minimized in high-speed printers, but the ability to do so is limited by the adverse effect on print wheel positioning accuracy of too much play in the splined shaft-to-gear interface. These concerns would be greatest if the print wheel motor were driven bidirectionally to minimize the angle of rotation to a desired character.
Both of the foregoing problems tend to worsen as the printer is used over a period of time; the gear teeth wear and the spline contacting surfaces of the movable bevel gear wear. These problems also become more critical as printing speed and the number of characters on a print wheel are increased. Angular errors that are relatively small, for example, 1.degree., in print wheels having small character sets, such as 36 characters spaced 10.degree. apart, become relatively large when the character set is increased, for example, to 96 characters spaced less than 4.degree. apart.
To overcome the foregoing objections, some printers currently on the market, such as the Wang Model 6581-W, utilize a DC SERVO motor, with sensor and feedback circuitry, mounted on the carriage to directly drive the print wheel. Far greater accuracy of angular positioning of the print wheel is thus made possible. However, it is obtained at the cost of greatly increasing the mass of the carriage. As a result, the speed at which the carriage can be stepped accurately along the print line from one print position to the next is limited.
U.S. Pat. No. 4,245,917 to Mosciatti, et al. seeks to minimize carriage mass by using instead of motors, a fixed linear magnet core mounting four separate armature coils for independent movement along the core to operate various printer elements. In general, this printer is asynchronous like Martin and Jensen, et al., but is much more complicated and heavier. Instead of bevel gears, Mosciatti, et al. use a rack and pinion which is also susceptible to backlash. They also use a photoelectric transducer to monitor print wheel position.
It would be preferable to obtain the angular accuracy of a direct drive step motor rotating the print wheel in either direction without the disadvantage of carrying the mass of such motor on the carriage. It would also be desirable to have the certainty, provided in synchronous printers, of knowing the relative position of the print wheel and carriage without the need for feedback circuitry, while having the flexibility of control of asynchronous printers. However, no known printer provides these capabilities or avoids all of the foregoing problems without introducing some other countervailing problem. Accordingly, there remains a need for an improved rotary serial printer.